Fix oversight in SANITIZE block resulting in NPE if there's only one read group.
Fix oversight in SANITIZE block resulting in NPE if there's only one read group.

alecw/picard,annkupi/picard,broadinstitute/picard,broadinstitute/picard,annkupi/picard,nh13/picard,broadinstitute/picard,alecw/picard,annkupi/picard,alecw/picard,nh13/picard,nh13/picard,annkupi/picard,alecw/picard,broadinstitute/picard,broadinstitute/picard,nh13/picard